"The leaders reiterated their grave concern over Russia's clear violation of international law and reaffirmed their support for Ukraine's sovereignty and territorial integrity," a White House summary of the phone calls said.

'Diplomatic path'

Downing Street confirmed the Prime Minister and President Obama agreed there was still an opportunity for Russia and its President Vladimir Putin to "resolve the situation diplomatically" by engaging in direct talks with the Ukrainians, returning Russian troops to their bases in Crimea and working with the rest of the international community to support free and fair elections in Ukraine in May.

In a statement, Mr Cameron's office said: "We should all work together to persuade President Putin to choose this diplomatic path out of the crisis.

"Both the Prime Minister and the President firmly believe the proposed referendum in Crimea would be illegal and any attempt to legitimise it would result in further consequences for Russia."

Russia effectively took control of Crimea - where its Black Sea Fleet is based - more than a week ago.

Crimea's pro-Russian leadership has set a date of 16 March for a referendum on joining Russia.

President Obama also held a conference call about the situation with Lithuanian President Dalia Grybauskaite, Latvian President Andris Berzins, and Estonian President Toomas Hendrik Ilves.

The White House said the leaders also agreed the referendum later this month will be a "violation of Ukraine's constitution", adding that all future decisions about the country should involve the government in Kiev.

David Cameron also held a separate call on the situation with Canadian Prime Minister Stephen Harper.

Downing Street said the two men discussed the need for the International Monetary Fund to lead on a package of financial assistance to the Ukrainian government to help provide economic stability.